To remove carbon-based particulates from exhaust gases emitted from diesel engines, ceramic honeycomb filters comprising porous ceramic honeycomb structures with both ends alternately sealed are used.
As shown in FIG. 1, a ceramic honeycomb structure 1 is substantially cylindrical (including an elliptic cross section) with partition walls 3 inside an outer wall 2 and a large number of cells (flow paths) 4 encircled thereby. As shown in FIG. 2(b), both ends of the flow paths 4 are alternately sealed by sealers 5a, 5b. 
The cleaning an exhaust gas by a honeycomb filter 10 is carried out as shown in FIG. 2(c). The exhaust gas 20a flows into the flow paths 4 of the honeycomb filter 10, passes through pores in the porous partition walls 3, and exits from the adjacent flow paths 4 as shown by 20b. Particulates in the exhaust gas are captured while passing through the pores of the partition walls 3. Particulates 30 are accumulated on an inner surface of each sealer 5 on an exit side. When particulates captured by the partition walls 4 exceed a permitted level, the clogging of the filter 10 occurs. Accordingly, the captured particulates are burned off by a burner or an electric heater to regenerate the filter 10.
Important for such particulates-capturing filter are filter characteristics such as pressure loss, particulates-capturing efficiency, breakage resistance and melting-away resistance, etc. Though the pressure loss can be reduced by increasing the porosity and pore size of partition walls or by decreasing exhaust gas resistance, larger porosity and pore size leads to lower strength in the partition walls, resulting in low breakage resistance of the filter. Further, sealers formed at both ends of the honeycomb filter not only increases the pressure loss, but also lowers the thermal shock resistance. It is thus difficult to satisfy both of the requirements of pressure loss and breakage resistance.
JP 7-332064 A discloses a method for connecting pores three-dimensionally in sealers on the side of discharging an exhaust gas so that its porosity is 110-140% of the porosity of partition walls, to prevent the pressure loss of a ceramic honeycomb filter from being increased by sealers. However, the ceramic honeycomb structure of JP 7-332064 A, as described in its Examples, has as small porosity as 45% and thus large pressure loss of a ceramic honeycomb filter. Because it does not have pores in sealers on an inlet side while it has pores in sealers on an exit side, an exhaust gas cannot pass through the sealers on the inlet side, resulting in insufficient effect of reducing the pressure loss.
JP 8-281034 A discloses that thermal shock at the time of regeneration is concentrated in boundaries between sealed portions and unsealed portions in the partition walls of a honeycomb filter, and that such boundaries (corresponding to seal depth) should not be continuous on a line to prevent the honeycomb filter from being broken by such thermal shock. However, when the seal depth of the honeycomb filter is nonuniform, there is only a small effective area as a filter in portions having large seal depth, resulting in large pressure loss. Also, with the nonuniform seal depth, filter areas differ from product to product, causing the problem that the resultant honeycomb filters do not have constant quality. Further, there is weak adhesion strength between the sealers and the partition walls in portions having small seal depth, the sealers are likely to peel off by the pressure of an exhaust gas or thermal shock, etc.
Usually, a ceramic honeycomb structure has square cells (flow paths) as shown in FIGS. 13(a) and (b), and the thickness of partition walls is substantially uniform throughout the ceramic honeycomb structure. A ceramic honeycomb structure having such structure has high strength in a direction in parallel with the partition walls, but its strength is low in a direction slanting to the partition walls. Accordingly, when used for catalyst converters and particulates-capturing filters, it is likely to suffer from cracking 13 in partition wall intersections by thermal shock or stress as shown in FIG. 6, resulting in breakage in a diagonal direction of cells.
To avoid such problems, JP 55-147154 A discloses a technology of making partition walls near an outer wall thicker than partition walls inside them to increase the strength of the overall ceramic honeycomb structure. However, because the partition walls are not thick in a core portion of the ceramic honeycomb structure, the partition wall intersections have uniform strength, so that cracking generated in the partition wall intersections continuously propagate through the core portion.
JP 51-20435 B discloses that partition wall intersections are arcuately or linearly expanded to prevent that cracking is generated in the partition wall intersections on which stress is concentrated, and that catalytic reaction efficiency decreases in corners of flow paths (facing partition wall intersections) in which an exhaust gas does not flow smoothly. However, because the strength of the partition wall intersections is uniform throughout the ceramic honeycomb structure, cracking generated by thermal shock or stress continuously propagates along the partition wall intersections.
JP 61-129015 A discloses a filter for cleaning an exhaust gas having partition walls whose pores are composed of small pores having a pore diameter of 5-40 μm and large pores having a pore diameter of 40-100 μm, the number of the small pores being 5-40 times that of the large pores. Though it does not describe porosity, the porosity is calculated as 43-64% from an accumulated pores volume of 0.3-0.7 cm3/g, assuming that cordierite has a true specific gravity of 2.5.
JP 61-54750 B discloses that by adjusting an open porosity and an average pore diameter, it is possible to design a filter from a high-capturing rate to a low-capturing rate. It describes that the preferred range of porosity is 33-90%.
Japanese Patent 2,578,176 discloses a porous ceramic honeycomb filter having a long particulates-capturing time, the porosity being 40-55%, and the volume of pores having diameters of 2 μm or less being 0.015 cm3/g or less.
JP 9-77573 A discloses a honeycomb structure having a high capturing rate, a low pressure loss and a low thermal expansion ratio, which has a porosity of 55-80% and an average pore diameter of 25-40 μm, pores in its partition walls being composed of small pores having diameters of 5-40 μm and large pores having diameters of 40-100 μm, and the number of small pores being 5-40 times that of large pores.
However, because these porous ceramic honeycomb filters have high porosity, they inevitably have low strength. In addition, because relatively flat powder such as carbon, graphite, etc. is used as a pore-forming material, pores have acute corners with large aspect ratios in their transverse cross sections. Therefore, stress concentration is likely to occur in the pores, causing decrease in the strength of the ceramic honeycomb structure. Thus, when it is used for particulates-capturing filters for exhaust gases from diesel engines, it is likely to be broken by thermal stress and shock, a mechanical fastening force at the time of assembling, vibration, etc.